In elementary/MS there were 2 black girls(full) i knew of with mbl-wl hair...one was relaxed and the other was a pressed natural. Both girls had nice hair but i was always partial to the pressed natural bc her hair was thicker.
I went to a high school which was probably 50/50 black/hispanic.
There were a good number of black girls with nice/healthy looking relaxed hair...but it wasn't mbl or wl...apl maybe the longest...but the hair would be really nice looking, thick, bouncy, full of body, shiny. I actually attribute the health and vibrant look to the frequent patronage of dominican hair shops. There were also black girls with chewed-up thin hair, apl or shorter.
The one black girl (full black, not mixed) at my high school who had waist length hair was a natural who pressed. All the hispanic girls (mostly dominicans) had waist length to butt length hair, all of 'em.
In college, the girls in my immediate circle (3), most had *decent* looking relaxed hair: The hair would be shoulder to apl, not particularly thick, but they didn't have see-through ends.
Grad school...the one black girl (there werent many of us) who had lengthy hair...it was about mid back, but she was also a natural (in the 3b range) who got her hair roller set at some dominican salon in Pa.
Ive moved back to nyc, and i don't see ANY black women with straight mbl to wl hair. All the 'black' women who have hair that long are noticeably mixed OR have locs.
Collectively, it would appear that black women do have the shortest hair. I attribute it to bad hair care practices AND genetics.